206 PROF. HUXLEY ON THE AGAMIC REPRODUCTION 



open close to the -^oilva. They consist of a delicate structureless coat lined by a thick 

 layer of granular substance, whose cellular composition is very indistinct in the fresh 

 state, but becomes obvious on the addition of acetic acid. 



The interior of the gland contains a clear, viscid, strongly-refracting substance, appa- 

 rently separated from the epithelial Kning by a membranous layer. I am in doubt, 

 however, whether this apparent membrane be anything more than the folded and wrinkled 

 outer layer of the viscid matter. ^AHien the Aphis is suddenly placed in glycerine or 

 subjected to slight pressure, a drop of the coUeterial secretion not unfrequently exudes 

 and manifests its viscidity by leaving a long trail. 



The fully-formed ovum (PL XL. fig. 1, P) measures about ^ih of an inch in length. It 

 is oval, rather smaller anteriorly, and of a deep green hue, in consequence of the colour of 

 the yelk. The chorion is a tough transparent membrane, about 9-oVotli of an inch thick, 

 and presents no external sculpturing or internal structm-e. Internal to the chorion is 

 a delicate vitelline membrane which immediately invests the yelk. It is, however, con- 

 nected with the chorion posteriorly. When the eg^ is heated with caustic potass, the 

 yelk is di'iven away from the sides (eventually dissolving), and with it the vitelline 

 membrane on the sides and at the anterior part of the ovum; posteriorly, however, I 

 always found it adherent. The yelk itself is very coarsely granular ; so that there would 

 be no chance of discovering the germinal vesicle, even if it existed. 



The recent observations of Leuckart and Meissner on the micropyle of the ovum in 

 Insects naturally induced me to look for such a structure m the egg of Aphis. 



Leuckart, in his elaborate essay, clearly shows that the micropyle may be single or 

 multiple, and may occur at either or both poles of the egg ; but unfortunately he gives us 

 less information respecting the ova of the Homopterous Herniptera than regarding those 

 of any other great group of Insecta. Cercopis, in fact, is the only genus of this division 

 in which he has observed the micropyle with certainty, and here there are two, one on 

 each side of the anterior pole. 



The anterior extremity of the chorion in Aphis (PI. XL. fig. 4, B) presents a smaU 

 conical papilla, in which I have been unable to discover any aperture. Internally, how- 

 ever, the corresponding surface of the chorion appears as it were rough and uneven ; and 

 when caustic potass is added, it, like the rest of the inner sui'face of the chorion, exhibits 

 a very curious marking, as if so many circles or more irregular figiu-es were impressed 

 upon it. The thickness of the jiapiUa is about 4 0^0 oth of an inch; and in young ova a 

 delicate fUiform appendage more than once appeared to be continuous with it : this, how- 

 ever, was invariably absent in fully-formed ova. 



At the opposite pole (fig. 4, A), the ovum presents a curious appendage, about -5^0 th of 

 an inch in length. "V^Tien the ovum is in its natural position within the ovary, the 

 epithelium of the latter, which closes over it below, leaves a sort of chamber in which this 

 appendage, ordinarily more or less closely applied against the chorion, is received. 



When the ovum is extracted, the appendage appears like a rope with loosened strands, 

 or a closely-plaited membrane, and is seen to be coated with a clear gelatinous substance, 

 in which many minute rod-like filaments of about joWth of an inch in length are im- 

 bedded. Treated with caustic potass, this clear substance and its imbedded particles are 



